To enable image density correction with positional errors of dots not only in a nozzle array direction, but in a perpendicular direction to a nozzle array reflected thereon.

A discrete dot image recorded as a test image by an ink jet head is read. A dot diameter R is obtained from the width of a predetermined density value of a density distribution in the nozzle array direction of the test image. An impact position Pyi in the nozzle array direction and an impact position Pxi in the perpendicular direction to the nozzle array are obtained from the center of gravity position of the density distribution, respectively. An impact position deviation in the perpendicular direction to the nozzle array is converted to an impact position deviation in the nozzle array direction (α*Pxi) and added to the impact position in the nozzle array direction. The impact position in the nozzle array direction is thus corrected. A total of cover areas of dots formed by a specific nozzle and adjacent nozzles of both sides is obtained from the corrected impact position in the nozzle array direction and the dot diameter in a nozzle pitch width section which includes the specific nozzle. A ratio of the total area and a predetermined reference value is calculated as data for image correction.